Broadband impedance-matching transformer

ABSTRACT

A balun transformer in which the characteristic impedance of each of its component transmission lines is selected to cause that line to function as a quarter wave transformer for providing an impedance transformation ratio for the balun which is other than an integral square.

iJited [72] Inventor Gilbert L. Boelke West Seneca, N.Y. [21] Appl. No. 850,470 [22] Filed Aug. 15, 1969 [45] Patented Oct. 19, 1971 [73] Assignee Sylvania Electric Products Inc.

[54] BROADBAND IMPEDANCE-MATCHING TRANSFORMER 3 Claims, 2 Drawing Figs.

[52] U.S. Cl. 333/26, 33 3/35 [51 Int. Cl. H03h 7/38, H03h 7/42 [50] Field of Search 333/25, 26, 35, 27

[5 6] References Cited FOREIGN PATENTS 811,351 4/1959 Great Britain 333/26 OTHER REFERENCES Primary Examinerl-lerman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorneys-Norman J. OMalley, Elmer J. Nealon and Edward J. Coleman ABSTRACT: A balun transformer in which the characteristic impedance of each of its component transmission lines is selected to cause that line to function as a quarter wave transformer for providing an impedance transformation ratio for the balun which is other than an integral square.

OUTPUT 2 22 BROADBAND IMPEDANCE-MATCHING TRANSFORMER BACKGROUND OF THE INVENTION The invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of the Army.

This invention relates to electrical transmission line devices and, more particularly, to broadband impedance-matching transformers.

It is an object of the present invention to provide an impedance transformation of arbitrary value which is effective over a large band of frequencies with high efficiency. In the prior art, conventional balun transformers were limited to applications requiring transfonnation ratios of an integral square, such as I, 4or 9, etc., at which the bandwidth of the balun device approaches infinity. Arbitrary impedance steps, i.e. including ratios other than integral squares, were obtained by the use of quarter wave transmission line transformers, for which the bandwidth approaches infinity at a ratio of 1:]. Quarter wave transformers, however suffer the limitation that as the transformation ratio increases, the bandwidth diminishes. Accordingly, arbitrary transformation ratios with improved bandwidth characteristics have been obtained in the past by cascading a balun and a quarter wave transformer. For example, an impedance step-up of 6may be provided by cascading a balun having a step-up ratio of 4with a quarter wave transformer having a step-up ratio of 1.5, thereby enabling the quarter wave transfonner to operate at near optimum bandwidth. Although providing an advantageous compromise for some applications, however, the cascading approach doubles the power loss of the impedance transformer as compared to using a balun or quarter wave transformer alone, as the power flows through both devices consecutively.

SUMMARY OF THE INVENTION With an awareness of the aforementioned disadvantages of the prior art, it is an object of the present invention to provide an improved impedance-matching transfonner.

It is another object of the present invention to provide an impedance transformer for providing an impedance transformation ratio other than an integral square with high-efficiency and broadband characteristics.

Briefly, these and other objects are achieved by am impedance-matching transformer comprising a plurality of quarter wavelength transmission lines connected as a balun between input and output terminals, wherein the characteristic impedance of said transmission lines is selected to cause each of the lines to function as a quarter wave transformer for providing an impedance transformation ratio for the balun which is other than an integral square.

BRIEF DESCRIPTION OF THE DRAWING This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 is a pictorial schematic of a balun transformer; and

FIG. 2 is a pictorial schematic of a quarter wave transformer.

DESCRIPTION OF PREFERRED EMBODIMENT For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

The present invention combines the impedance transformation capabilities of the balun and quarter wave transformer into a single balun device to thereby provide impedance transformation ratios other than integral squares with improved bandwidth, as compared to a quarter wave transformer, and improved efficiency, with respect to cascaded sections. More specifically, a bandwidth is obtained which is substantially the same as that provided by an optimum compromise of two cascaded devices, but with half the power loss.

FIG. 1 shows a typical two section balun transfonner which conventionally provides a 1:4 impedance transformation. Of course, this illustration is merely representative, as the balun may comprise any plurality of transmission lines, depending upon the transformation ratio desired. For example, a conventional 9:1 balun preferably comprises three transmission lines. Referring to FIG. 1, the illustrated transformer comprises a first quarter wavelength section of coaxial transmission line 10, having an outer conductor 14 and an inner conductor 16, and a second quarter wavelength transmission line 12, having an outer conductor 18 and an inner conductor 20. Transmission lines 10 and 12 are connected as a balun transformer between coaxial input and output terminals, 22 and 24 respectively, each having outer shields connected to ground. More specifically, input terminal 22 is connected in parallel to center conductors 16 and 20 at the input ends of the lines; outer conductors l4 and 18 are interconnected at the input ends of the lines by conductor 26; output terminal 24 is connected to center conductor 16 at the output end; the output end of outer conductor 14 is connected by means of conductor 28 to the output end of center conductor 20; and the input and output ends of outer conductor 18 are connected to ground, or any other suitable source of reference potential. In eflect, therefore, the transmission lines are parallel connected to input terminal 22 and serially connected to output terminal 24. Terminal 22 is adapted to be connected to an input circuit (not shown) having an impedance 2,; terminal 24 is adapted to be connected to an output circuit (not shown) having an impedance Z and each of the transmission lines I0 and 12 have a selected characteristic impedance 2,. In other words, the balun is designed to provide a load of Z, at input terminal 22 and to match with a load impedance of 2, at output terminal 24.

In a conventional balun of the type shown in FIG. 1, 2 /2 ,=4 and Z,,-=2 Z,.

In accordance with the present invention, however, an impedance transformation ratio 2 /2 between terminals 22 and 24 which is other than an integral square is provided by selecting the characteristic impedance 2 of the balun to cause each of the transmission lines 10 and 12 to function as a quarter wave transformer. More specifically, each of the balun transmission lines 10 and 12 may be viewed as a quarter wave transformer, as shown in FIG. 2. This representative quarter wave transformer is shown comprising an outer conductor 30, each end of which is connected to ground, and a center con ductor 32, having representative input and output terminals 36 and 38. As the two balun transmission lines 10 and 12 are connected in parallel at terminal 22, each line section must provide a load impedance which is twice the overall balun load impedance of 2,; hence, the input or load impedance Z, at terminal 36 of the FIG. 2 quarter wave transformer may be represented as Z,,=2 Z,. Further, since the coaxial lines 10 and 12 are connected in series to add up to the output impedance of 2,; the termination 38 of the FIG. 2 quarter wave transformer may be represented as having an impedance Z,,=Z-,l2 As Z,,=Z /Z,, for a quarter wave transformer, the characteristic impedance 2,, for such a transformer is determined from the formula Z,,= T. It will be noted, however, that by substitution, Z,,= [(2 2,) (Z /2)== /Z, 2,. Hence, the characteristic impedance Z, for the balun transformer of FIG. 1 may be determined from the same equation, with respect to its input and put impedances, as a quarter wave transformer. As a result, selection of 2,, in this manner enables obtaining an arbitrary transformation ratio 2 /2, for a balun transformer, thereby providing advantages of lower loss, improved bandwidth and flexibility over prior art techniques.

For example, say a 1:6 impedance transformation from 8.33 to 50 ohms is desired. As the balun input impedance Z,=8.33 ohms, each of the input transmission lines 10 and 12 must provide a load of 2,, =1 6.66 ohms, such that the parallel combination is 8.33 ohms. At the balun output, the series connected transmission lines 10 and 12 must add up to 50 ohms; thus, each line must be terminated in Z =25 ohms. Accordingly, 2,,

F,= [8.33-50=20.4 ohms. Hence, if the balun is constructed of two quarter wave length transmission line sections each having a characteristic impedance of 20.4 the overall transformation will meet the requirements of 1:6 into a SO-ohm load.

The loss factor of the arbitrary" balun is half that of a cascaded combination of conventional devices as the signal traverses only half the length of the transmission line. Further, the invention provides circuit simplication with attendant size reduction. The transformer can be applied to transistor and vacuum-tube amplifiers, varactor or other RF device circuits, antenna systems, and passive RF devices such as hybrids, circulators and isolators requiring impedance transfon-nations. The invention may be embodied in a balun comprising any lurality of transmission lines of any type, the formula Z,,=

%,Z, being equally applicable.

What is claimed is:

1. An impedance matching transformer for providing a transformation ratio other than an integral square between an input circuit having an impedance Z, and an output circuit having an impedance 2 comprising, a first terminal adapted to be connected to said input circuit, a second terminal adapted to be connected to said output circuit, and a plurality of transmission lines connected as a balun transformer between said first and second terminals, each of said transmission lines being substantially an odd multiple of a quarter wavelength and having a characteristic impedance equal to 2. A transformer in accordance with claim 1 wherein said plurality of transmission lines comprises first and second transmission lines each having a center conductor and an outer conductor, said first terminal is connected in parallel to the center conductors of said transmission lines at the input ends thereof, the outer conductors of said transmission lines are interconnected at the input ends thereof, said second terminal is connected to the center conductor of said first transmission line at the output end thereof, the output end of the outer conductor of said first transmission line is connected to the output end of the center conductor of said second transmission line, and the input and output ends of the outer conductor of said second transmission lines are adapted to be connected to a source of reference potential.

3. An impedance matching transformer for providing a transformation ratio other than an integral square comprising, an input terminal, an output terminal, and a plurality of quarter wavelength transmission lines parallel connected to said input terminal and serially connected to said output terminal, the characteristic impedance of said transmission lines being selected to cause each of said transmission lines to function as a quarter wave transfonner. 

1. An impedance matching transformer for providing a transformation ratio other than an integral square between an input circuit having an impedance Z1 and an output circuit having an impedance Z2 comprising, a first terminal adapted to be connected to said input circuit, a second terminal adapted to be connected to said output circuit, and a plurality of transmission lines connected as a balun transformer between said first and second terminals, each of said transmission lines being substantially an odd multiple of a quarter wavelength and having a characteristic impedance equal to Z1Z2.
 2. A transformer in accordance with claim 1 wherein said plurality of transmission lines comprises first and second transmission lines each having a center conDuctor and an outer conductor, said first terminal is connected in parallel to the center conductors of said transmission lines at the input ends thereof, the outer conductors of said transmission lines are interconnected at the input ends thereof, said second terminal is connected to the center conductor of said first transmission line at the output end thereof, the output end of the outer conductor of said first transmission line is connected to the output end of the center conductor of said second transmission line, and the input and output ends of the outer conductor of said second transmission lines are adapted to be connected to a source of reference potential.
 3. An impedance matching transformer for providing a transformation ratio other than an integral square comprising, an input terminal, an output terminal, and a plurality of quarter wavelength transmission lines parallel connected to said input terminal and serially connected to said output terminal, the characteristic impedance of said transmission lines being selected to cause each of said transmission lines to function as a quarter wave transformer. 